Can a bubble study result in a false positive?

False Positive Bubble Studies in Echocardiography

Yes, false positive bubble studies can occur during echocardiographic evaluation for right-to-left shunts, though they are relatively uncommon when proper technique and interpretation are employed. 1, 2, 3

Mechanisms of False Positive Results

False positive results in bubble studies can occur through several mechanisms:

1. Technical and Procedural Factors:

   • Inadequate agitation of saline solution leading to larger bubbles that may cross pulmonary circulation
   • Improper timing of image acquisition relative to contrast injection
   • Misinterpretation of normal intracardiac structures or artifacts

2. Physiological Factors:

   • Pulmonary shunts misinterpreted as cardiac shunts: Intrapulmonary shunts (such as pulmonary arteriovenous malformations) can allow bubbles to pass through the pulmonary circulation and appear in the left heart 3
   • Timing of bubble appearance: Bubbles appearing in the left atrium after 3-5 cardiac cycles typically indicate pulmonary rather than cardiac shunting, but this distinction may be missed 3

3. Anatomical Variations:

   • Congenital abnormalities that may mimic shunts
   • Valsalva effect causing transient changes in intracardiac pressures 3

Diagnostic Accuracy Considerations

The transesophageal echocardiogram (TEE) is preferred over transthoracic echocardiogram (TTE) for bubble studies due to superior visualization of cardiac structures. However, even TEE may produce false-negative results if vegetations are small or have embolized 1. This suggests that false positives, while less common, are also possible due to technical limitations and interpretation challenges.

Clinical Implications

False positive bubble studies can lead to:

• Unnecessary additional testing
• Inappropriate patient management decisions
• Potential psychological impact on patients
• Unnecessary referrals for invasive procedures or interventions 4

Minimizing False Positive Results

To minimize false positive results:

• Proper technique: Ensure adequate agitation of saline to create microbubbles of appropriate size
• Standardized protocols: Follow established protocols for bubble study performance 2
• Correct timing assessment: Carefully assess the timing of bubble appearance in the left heart chambers
  • Immediate appearance (1-3 cardiac cycles): Suggests intracardiac shunt
  • Delayed appearance (>3-5 cardiac cycles): Suggests intrapulmonary shunt 3
• Confirmatory testing: Consider confirmatory testing with alternative imaging modalities when results are equivocal

Safety Considerations

While bubble studies are generally safe, rare complications can occur. Studies have reported cerebral ischemic events associated with bubble studies, particularly in patients with right-to-left shunts 5. The reported complication rate is approximately 0.1%, which warrants informed consent before the procedure 4.

Interpretation Guidelines

When interpreting bubble study results:

• Consider the clinical context and pre-test probability
• Assess timing of bubble appearance in left heart chambers
• Evaluate the quantity of bubbles crossing into the left heart
• Correlate with other imaging findings
• Consider alternative explanations for positive findings

Conclusion

False positive bubble studies do occur in clinical practice. Understanding the mechanisms, implementing standardized protocols, and careful interpretation can help minimize their occurrence and impact on patient management. When results are equivocal or unexpected, confirmatory testing should be considered before making definitive clinical decisions.